Methylamines are generally prepared commercially by continuous reaction of methanol and ammonia in the presence of a dehydration catalyst such as silica-alumina. The reactants are typically combined in the vapor phase, at temperatures in the range of 300.degree. C. to 500.degree. C., and at elevated pressures. Trimethylamine is the principal component of the resulting product stream accompanied by lesser amounts of monomethylamine and dimethylamine. The methylamines are used in processes for pesticides, solvents and water treatment. From a commercial perspective, the most valued product of the reaction is dimethylamine in view of its widespread industrial use as a chemical intermediate (e.g., for the production of dimethylformamide). Thus, a major objective of those seeking to enhance the commercial efficiency of this process has been to improve overall yields of dimethylamine and monomethylamine, relative to trimethylamine. Among the approaches taken to meet this goal are recycling of trimethylamine, adjustment of the ratio of methanol to ammonia reactants and use of selected dehydrating or aminating catalyst species. Many patents and technical contributions are available because of the commercial importance of the process. A summary of some of the relevant art for methylamine synthesis using zeolite catalysts is disclosed in U.S. Pat. No. 5,344,989 (Corbin et al.).
Zeolites chabazite, where the zeolite is derived from mineral sources and the silicon to aluminum ratios in said zeolites is less than about 2:1, as well as zeolites rho are known to be useful as catalysts for methylamines. See U.S. Pat. No. 5,569,785 (Kourtakis et al.) and references cited therein. The use of natural, H-exchanged and M-exchanged chabazites, where M is one or more alkali metal cations selected from the group consisting of Na, K, Rb and Cs is disclosed in U.S. Pat. No. 4,737,592 (Abrams et al.).
U.S. Pat. No. 5,399,769 (Wilhelm et al.) discloses an improved methylamines process using synthetic chabazites as catalysts. Runs 3-5 in Table 5 show the methylamines distribution for different synthetic chabazites with a Si:Al ratio of about 2.5:1. The molar ratio of ammonia to methanol was 3.5: 1; such an excess of ammonia is known to decrease trimethylamine formation. The percentage of dimethylamine shown for each run was 26, 48.7 and 51.5, respectively.
What are needed and are of significant interest to the chemical industry are process improvements which suppress production of trimethylamine and optimize dimethylamine and monomethylamine yields. Other objects and advantages of the present invention will become apparent to those skilled in the art upon reference to the detailed description which follows hereinafter.